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Impulse and Momentum

Impulse and Momentum. How can you karate-chop through a stack of wood? Why is follow-through important when swinging a club, racket, or bat? When a sports announcer says a team has momentum- what do they mean?. What is Momentum?.

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Impulse and Momentum

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  1. Impulse and Momentum How can you karate-chop through a stack of wood? Why is follow-through important when swinging a club, racket, or bat? When a sports announcer says a team has momentum- what do they mean?

  2. What is Momentum? • Harder to stop a truck than a skateboard- truck has more momentum • Momentum=inertia in motion momentum=mass x velocity p=mv

  3. What makes momentum big? • You can have large momentum if you have…? • Large mass • Large velocity • Can you think of a way for the truck and the skateboard to have the same momentum?

  4. Momentum in a collision • Think of hitting a baseball- what is the momentum of the bat and ball before the collision? • What is the momentum of the bat and ball after? • Draw vector diagram of each moment- yes…momentum is a vector

  5. Problem solving- momentum • Determine the momentum of a 60kg player moving eastward at 9m/s p = m*v = 60 kg*9 m/s p = 540 kg•m/s, east

  6. Problem solving- momentum • Determine the momentum of a 1000kg car moving southward at 20m/s p = m*v = 1000 kg*20 m/s p = 20 000 kg•m/s south

  7. Problem solving- momentum • A car possesses 20 000 units of momentum. What would be the car's new momentum if ... a. its velocity were doubled. b. its velocity were tripled. c. its mass were doubled (by adding more passengers and a greater load) d. both its velocity were doubled and its mass were doubled.

  8. Solutions- Momentum A. p = 40 000 units (doubling the velocity will double the momentum) B. p = 60 000 units (tripling the velocity will triple the momentum) C. p = 40 000 units (doubling the mass will double the momentum) D. p = 80 000 units (doubling the velocity will double the momentum and doubling the mass will also double the momentum; the combined result is that the momentum is doubled twice -quadrupled)

  9. Impulse • If the momentum of an object changes, either its mass or its velocity must change • Since we don’t usually change the mass of objects, what must have happened?

  10. Impulse • Remember Newton’s 2nd Law? • If the velocity changed, there was acceleration, then there must have been a force • How long that force acted is also important Impulse=Ft= force x time Impulse is the change in momentum so: Ft=(mv) This is called the impulse-momentum theorem

  11. Increasing momentum • To increase momentum the most, what do we need to do? • Apply the largest force for the longest period of time • Thus the concept of “follow through”

  12. Decreasing momentum • A volunteer??? • Come up to the front of the class and catch this falling mass with your hand. • You will apply a force over a certain time (impulse) to change the momentum

  13. Decreasing Momentum • Now- place the back of your catching hand on the lab table and I will drop the mass again • What will happen? • Why? • Write the impulse-momentum theorem and use it to explain

  14. Questions??? • When you jump, why do you bend your knees upon landing? • In boxing, why do you “ride” with the punch? • When a dish falls, would the impulse be more if it falls on concrete than if it falls on carpet? No- the change in momentum is the same so the impulse is the same!

  15. Conservation of Momentum • In the absence of any external force, the momentum of a system remains unchanged • Forces internal to the system do not matter

  16. What does it mean to be a system? • In an isolated system, no net external forces are acting • Think of financial systems…if Jack has $100 and gives $50 to Jane, both would have $50 and the total amount of $100 in the system is conserved.

  17. Examples • What is the momentum before? After? • Which moves faster, the cannon or cannonball- why?

  18. Examples

  19. Elastic collisions • When objects collide without being permanently deformed or generating heat • Perfect “bounce”

  20. Inelastic Collisions • Colliding objects stick together after the collision • Could have permanent deformation or heat generation • MOMENTUM IS STILL CONSERVED!

  21. Outside forces • Most collisions will involve some outside force- friction, air resistance, etc • BUT- these forces generally small enough to have conservation of momentum

  22. Problem-Solving • A 6kg fish swims 1m/s towards and swallows a 2kg fish that is rest. Momentum is conserved. What is the velocity of the fish after lunch?

  23. Solution • Momentum before=momentum after • m1v1=m2v2 • (6kg)(1m/s)+(2kg)(0m/s)=(6kg+2kg)v2 • v2=6/8=0.75m/s

  24. Try on your own…beware of signs!!!!! • What if the little fish wasn’t still but was actually swimming away from the big fish at 0.5m/s when he got swallowed. What was the velocity after lunch? • What if the little fish was really stupid and was swimming towards the big fish at 2m/s when he was swallowed- what was the velocity after lunch?

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